Follow-up to the salvaged registration contract:
- share one _raise_if_cron_registration_error() helper for the two
byte-identical dashboard 424 except-blocks (web_server + cron router,
via the existing late() seam)
- add endpoint-level 424 coverage for /api/cron/blueprints/instantiate
(previously only the sync worker was tested)
- give chat/CLI surfaces a human-facing user_message() (job name, no
exception class name) and add a recovery hint (pause/resume or update
re-registers via provider reconcile) to the model/REST message
- consolidate five inline provider test doubles into one ABC-subclassing
make_cron_provider conftest factory; the web_server test double now
subclasses CronScheduler so an ABC rename fails loudly
- narrow the wrapper facade to keyword-only (**kwargs) and route the
tool's partial-failure return through tool_error()
kshitij